1. Field of the Invention
The present invention relates to a method of manufacturing electrophoretic display (EPD) having an organic thin film transistor (OTFT) control circuit and an EPD manufactured using the method.
2. Description of the Related Art
An electrophoretic display (EPD) is a new type of reflective display, whose image formation mechanism relies on the alternating arrangement of charged coloured particles or fluids in a dispersion or multiphase system by applying an electric field. Examples of EPDs include particle systems of E-Ink and Sipix, “Liquid Powder” of Bridgestone, and “Electrowetting” of Liquavista. These EPDs are all capacitively controlled, i.e., they include a cell that can be capacitively switchable in the optical characteristic (or performance) of the cell. For capacitively switching, a pixel electrode is assigned to each of the cells. When applying a voltage to the pixel electrode, the optical characteristic (or performance) of one or multiple cells can be affected. To describe such operations, an EPD having a particle system will be described in more detail. However, the present invention is not thereby limited, and can be directed to all capacitively switchable EPDS.
An EPD includes a plurality of single EPD units. The EPD unit includes at least one cell filled with charged particles dispersed in a suitable solvent in terms of simplicity of design. For capacitively switching, at least one pixel electrode is assigned to each cell, wherein the pixel electrode is designed and arranged so that charged particles are aligned in the direction of an electric field when applying a voltage to the cell. Likewise, the switching mechanism is capacitive, and the charged particles move in the direction of the pixel electrode and attach thereto in the presence of an electric field. When the accumulated coating is conditioned, it is substantially not transparent to light. In contrast, the dispersion with freely distributed particles allows light to transmit. This realisable technology allows the production of highly flexible displays, such as electronic newspapers. In addition, this technology can be applied to an image forming component in a plurality of electronic products, for example, laptops or mobile phones.
In one embodiment, the plurality of EPD units of the EPD are arranged on a common substrate. Such component of the EPD is referred to as an EPD module. The pixel electrode is applied to the EPD module. To control the operation of the plurality of EPD units of the EPD module, a complex control circuit needs to be provided.
Small-sized EPDs can be controlled via a passive matrix (PM) scheme. That is, a particular pixel can be accessed by applying a voltage to a particular electrode (or conductor) arranged in a row and a particular electrode (or conductor) arranged in a column. For large-sized displays, this PM scheme is not suitable.
In large-sized displays, an active matrix (AM) scheme is used. Here, each pixel is individually addressed to, e.g., transmit light or not transmit light via at least one switch. Likewise, each single pixel includes an active amplifier, for example, in the form of a thin film transistor (TFT). The amplifier which is assigned to each pixel has two important functions. That is, the control voltage of a pixel decreases with increasing number of the rows and columns of a pixel matrix being used (which limit the size of a device driven by the PM scheme). However, the amplifier can increase this voltage to the value required for switching the pixel (or cell). But, the continuous increase of the number of columns and rows and reduction of the size of pixels lead to an increase of the parasitic capacities of the pixel. To reach the high frame rate which is often necessary, the capacity should not induce reduction of the switching speed. A local amplifier with separate power supply can allow for the fast switching with short powerful current pulses.
By virtue of the discovery and development of organic semiconductor materials, organic thin film transistors (OTFT) have been spotlighted for the development of miniaturised electronic components. An OTFT includes a conductive gate electrode. The conductive gate electrode is covered with a thin dielectric film to which an active organic semiconductor material is attached. Normally, minor molecules and oligomers such as pentacene and polythiophene are used as a semiconductor material. The semiconductor layer has a thickness of several tens of nm and is laterally limited by the source and drain electrodes. Longitudinally, the semiconductor layer has a size of several hundreds of nm. Ideally, the organic semiconductor material is available as a monocrystal, but polycrystalline or amorphous films can also be used due to their significantly cost-effective production.
The production of OTFTs can be very cost-effective, e.g. in term of already existing technologies to produce synthetic micro structures, e.g., suitable inkjet printing technologies. That is, the OTFTs can be manufactured by utilizing injet printing to produce organic semiconductor layers. The ink is selectively applied to a definite area of the substrate, and the ink is composed of the organic semiconductor (or the organic semiconductor is contained in the ink). According to an embodiment of the present invention, the pixel electrodes and the OTFT layers are applied to an EPD module. That is, in one embodiment, the EPD module and the OTFT module including pixel electrodes are laminated together.
In addition, EPD modules are commercially available from different manufacturers. To produce the EPD modules that can directly operate, the EPD module needs to include a control circuit suited to a pixel electrode, in particular an OTFT control circuit. To this extent, an embodiment of the present invention uses an OTFT module, which is connected to the EPD module having applied pixel electrodes by using a laminating method. The OTFT module includes a plurality of OTFT units, which are arranged on a common substrate. Here, both modules have to be mutually aligned. By arranging both modules in such a way, the pixel electrodes are electrically connected desirably to establish a plurality of EPD units, thereby establishing the OTFT units controlling the operation of the plurality of EPDs. This requires structure which supports a proper mutual alignment of the modules. The creation of such structures as well as the process of aligning both modules increase manufacturing costs and are error-prone which can increase defects.
When manufacturing the OTFT control circuit on a separate substrate film, more problems occur as follows:
(1) During the process, there are always some temperature steps which lead to an uncontrollable deformation and expansion of the substrate film. This slightly deformed film can then be combined with the EPD module only imprecisely, i.e., yield, quality and reproducibility of the manufacturing process are reduced.
(2) When laminating the EPD module with the OTFT module, additional pressure is required. This pressure can damage the sensitive transistors.
In addition, laminating generally involves application of an adhesive agent between the modules as well as a thermal treatment for hardening of the adhesive agent. These laminating measures are linked with material costs and extra processes and can result in further defect which might increase manufacturing defectives. Also, the organic semiconductor layers of the OTFT are temperature sensitive and can be limited in functionality by a thermal treatment.